Phonograph



Aug. 29, 1944. s. A. SCHERBATSKOY PHONOGRAPH Filed Oct. 30, 1943 4* Alma(MW i'liii V Ii'III! FIG. 5

Patented Aug. 29, 1944 UNITED STATES PATENT OFFICE PHONOGRAPH SergeAlexander Scherbatskoy, Tulsa, Okla. Application October 30, 1943,Serial No. 508,357

(Cl. I'm-100.4)

3 Claims.

This invention relates to phonographicreproduction and more particularlyto the picking up and reproducing of oscillations from an undulatingrecord groove. By way of preferred example, the invention has beenillustrated and described with reference to grooves undulatinglaterally, or in the plane of the record surface, but it is to beunderstood that no unnecessary limitation thereto is intended.

It is an object of the invention to provide a generally improved pick-updevice.

It is an object to provide a phonograph apparatus of extreme simplicity,ruggedness and lightness by providing a new type of pick-up device whichsimplifies considerably the structure of the whole phonograph assembly.

It is an object toprovide a pick-up device of extreme simplicity andruggedness which operates satisfactorily with a very low pressure of itsneedle or other stylus against the record. With similar objects in view,a number of pickup devices of this nature have been used in the past.These may be considered as comprising a moving or armature systemsubjected to vibration by the undulations of the record groove (whichwill be understood to be moved relatively to the pick-up device), and atranslating system responsive to that vibration and serving to translateit into electric oscillations. The translating system may, for example,have a sensitiv portion in relatively close spaced relationship to thearmature, the vibration of the armature serving to oscillatorily varythis spaced relationship so to influence the translating system. Inanother view, the translating may have a. field, for exampleelectrostatic or electromagnetic, in which the armature vibrates, theresulting oscillatory alteration of the field giving rise to the desiredelectric oscillations.

The pick-up systems heretofore used where mostly of electrostatic,electromagnetic or piezoelectric type in which the translation of thearmature oscillations into electrical current utilized the effects ofelectrostatic or electromagnetic induction or the piezoelectric efiect.It is well known that in the pick-up systems of the prior type theconversion of the mechanical vibrational energy of the pick-up needleinto electrical energy took place without the presence of any externalsources of energy. Consequently, if we neglect any frictional and eddycurrent losses, we will realize that the electrical energy derived fromthe pick-up systems was substantially equal to the mechanical energy ofthe vibrations of the needle. It is apparent that this electrical energywas too weak to actuate a loudspeaker, and, consequently, in order toprovide a successful phonographic reproduction of speech and music, itwas necessary to provide electronic amplification in order to intensifythe currents derived from the pick-up to a degree required for theoperation of a loudspeaker.

Consequently, the phonograph system used in the prior art required anelectronic amplifying equipment which rendered the phonographscumbersome, complicated, anad expensive.

It is a purpose of the present invention to provide new phonographicsystems in which electronic amplification is eliminated which combineruggedness and simplicity with low cost and which,'therefore, possessconsiderably increased market possibilities. I am accomplishing thispurpose by providing the phonograph with a new pick-up device ofelectrolytic type in which the conversion from the mechanical,vibrational energy of the needle'into electrical energy takes place inpresence of a supply of current, such as an electric battery.Consequently, in my new t of, pick-up device, the electrical output isconsiderably larger than the vibrational energy of the needle and mayincrease at will by increasing the current supply of the battery.Therefore, the electronic amplification is no longer necessary and thewhole phonograph assembly is of a considerable simplicity in design.

It is the purpose of my invention to provide a new type of aphonograph-1c pick-up utilizing an electrolyte and two electrodesimmersed in the electrolyte and in which the distance between theelectrodes is varied by the vibrations of the pick-up needle. Such apick-up device will be included in a suitable electrical networkenergized by a source of electrical energy and across the outputterminals of the electrical network electrical currents will be obtainedwhich will correspond to the vibrations of the pick-up needle. It is afurther purpose of my invention to provide an improved form ofconstruction of vibrational resistances, of the type referred to. Thisconstruction relates particularly to vibrational electrolyticresistances in which the distance between the electrodes can be variedwhen the device is exposed to the vibrations that it is desired toreproduce.

In general, it is an object of my invention to provide a phonographicpick-up device for translating mechanical vibrations into electricalcurrents which involves the principle of varying the distance betweenelectrodes immersed in an electrolyte. Such adevice may be constructedin an efllcient and compact'manner similar to well known other pick-updevices for translating mechanical vibrations, into electrical currents.1

Other objects and advantages of the invention will be apparent from thefollowing descriptions when read in conjunction with the drawing, inwhich:

Figure 1 is a view, partly elevational and partly cross-sectional, of apick-up device in which my invention has been embodied together with aschematic diagram of the remaining associated apparatus of thephonograph equipment.

Figure 2 is a cross-sectional view taken along the line 2-2 of Figure 1.

Figure 3 is a cross-sectional view taken along the line 3-3 of Figure 2.

Figure 4 is a cross-sectional view taken along line 4-4 or Figure 2.

Figure 5 shows a modified portion of the phonograph equipment which isenergized by a source of alternating current.

Referring now more particularly to Figure 1, there is showndiagrammatically a phonograph system including a pick-up device itprovided with a vibratable needle l5 subject to vibration of the recordgroove I! (which is understood to be moved relatively to the pick-updevice). The pick-up device is of a novel type and consists of anelectrolytic cell formed by a casing i i, containing a suitableelectrolyte l2 and two electrodes submerged in the electrolyte, one ofsaid electrodes being movable and consisting of the needle 55 and theother electrode being stationary and consisting of a metallic spring itwhich shall be referred to later in the description.

The casing it may be secured at the front extremity of a cylindrical arm23 with a slight inclination to bring its bottom somewhat more forwardthan its top. The arm will be understood to b supported at its rearextremity by a support l9. The support l9 renders the forward extremityof the arm free to move up and down forengagement of the pick-up needlei5 with the record I1, and from side to side so that the needle maytrack a groove in the record :siderably intensified form. The output ofthe tight to the closure member 3L Through theresilient block 35, forexample centrally thereof,

there is provided a very small diameter hole 33.

To the inside of the top of the casing ll there may be secured ametallic block 31 and in the bottom surface of this block, in line withthe hole 36 abovementioned, there may be provided a concave conicalbearing 38. The armature system oi! the pick-up device comprises theneedle ll having the record-contacting point 39 at its bottom extremityand the bearing-contacting point 40 at its top extremity. The needle issimply being pushed upwardly through the hole 36 until its extremity 40engages the bearing 38. The hole 36 is readily arranged to have adiameter sufliciently small'so that the inserted needle will be securelyretained water-tight.

It will be understood that the groove undulations in the moving recordI! (with its direction of movement under the needle indicated by the theresilient block 35 in immediate contact therer with). Not only will thisfrequency inherently be very high, "but also the tendency to excessivevibration due to this resonance is effectively damped by theblock 35;-For this damping action as well as for such damping'action as may bedesirable on the intended vibration of the neecontained within thecasing H,

network 23 is translated into sound by means of a loudspeaker 30 of a cventional design.

Referring now more articularly to the pickup device In, the metalliccasing H is shown as having a closure member 3| removably secured acrossits otherwise open bottom. Through the bottom member 3| there isprovided a relatively large hole 32; the very top portion 33 of which ismade of reduced diameter to provide a shoulder.

In the hole 32 there is retained a circular block 35 of resilientmaterial. While this retention may be insured in any desired manner, Ihave by way of example illustrated the hole 32 as formed with a slightflange 34 inwardly directly at its bottom; the block 35, compressedduring insertion to pass the flange, thereafter expands die, it may bedesirable to choose the material of the block 35 to have a significantviscous damping, as well as resilient, characteristic. As an example ofan appropriate material there may be mentioned the synthetic rubbercurrently available as neoprene.

The translating system which I have illustrated is an electrolytic one,basically comprising two electrodes in special relationship one from theother and submerged in an electrolytic solution One of the twoelectrodes is stationary and consists of the spring IS. The otherelectrode is the vibratory needle I 5 which with-the stationary oneforms an oscillatorily variable electrolyte resistance. As a mountingfor the stationary electrode there may be secured to the inside surfaceof the righthand side of the casing H block 4| of insulating material(such for example as the phenol-resin composition known as Bakelite"),which may extend very nearly down to-the bottom member 31.

Just above the bottom of this block 4| there may be provided ahorizontal threaded hole 42 extending in a side-to-side directionthrough the block, and accessible from outside the casing by virtue ofan aligned, slightly larger-diameter hole provided in the casing. Inthis hole 42 whose axis intersects the needle l5 slightly above theresilient block 35 there may be provided an adjustable screw 43. Thisscrew is employed for the adjustment of the stationary electrode IIwhich is then made electrically independent of the screw (so that, amongother things, touching of the screw with a screwdriver for adjustmentpurposes will not cause undue electrical disturbances). Accordingly, thestationary electrode has a form of a spring it having its upperextremity secured to the inner surface of the block 4! near the top ofthe latter, extending there- .from diagonally downwardly and toward theneedle, and finally curving into a lowerportion essentially parallel toand near the needle. A small pad 45 of insulating material may-besecured on the outer side of spring Ilopposite the end of the screw 43,and the spring It may be tensioned to maintain contact of this padagainst that screw end. It will thus be understood that the screw 42will serve to adjust the spacing of the operatively stationary electrodeor spring I8 from the needle or vibratory electrode I5.. In,.

general, this spacing will ordinarily be adjusted to theminimum whichstill insures failure-of contacting of the two electrodes underconditions of maximum vibration of II.

Preferably the arm will be secured to the casing II so that its axisintersects the needle I5. This may be offset (e. g., to the left) fromthe center of the casing, in view of the space requirement which existson the one side for the sta- .tionary electrode and its support, andwhich has no counterpart on the other side. Any tendency towardside-to-side unbalance, to such extent as 20, through the bearing member31 which rests" on its upper extremity. Furthermore, the arm 20 iselectrically connected through the mounting of the arm Is to the outputterminal 22. For

electrical connection to the stationary electrode I6 there may beemployed a conductor 41.; this may pass longitudinally through thecylindrical arm 20 whose forward extremity preferably is open into theinterior of the casing I I. The portion of conductor 41 within the armmaybe insulated by the usual insulating covering 48. Further, in orderthat the capacity between it and the arm may be kept both constant andat a minimum, it is preferable that it be held centrally of the arm by apacking 49 of suitable insulating fibrous material or the like.

In considering the action of the structure of the electrolytic pick-upit is convenient to bear in mind that the eiiiciency of the translatingsystem and action is dependent on the spacing of the armature or needleI5 fromv the stationary electrode or spring It and upon the electrolyteresistance of the liquid in which both electrodes are submerged. It iswell known that the electrolytic resistance between the electrodesdepends upon the resistivity of the electrolyte which is a function offactors such as the nature of the electrolyte, its concentration, etc.,and also dependsupon the distance between the electrodes. Consequentlyunder the aforementioned conditions when no vibrations are present theelectrolyte resistance between the electrodes. I5 and I6 has a fixed anddetermined value, because the distance between the electrodes I4 and I8is fixed and determined.

It will be understood that as the armature or needle I5 is vibrated bythe groove undulations, the distance between the needle I5 and thespring gree of unbalance of the bridge.

to vibrations by the undulations of the record groove.

The output leads 2| and 22 of the pick-up device are connected to aWheatstone bridge circuit 50 which is energized by a D. C. source oivoltage, which is represented by a battery 24. One arm of the Wheatstonebridge consists of the electrolytic resistance inserted between theleads 2|, 22, and the remaining three arms of the bridge circuitconsists of resistors 58, 51, and 58.

The battery 24 is applied-between the terminals 50 and BI, the terminal50 being the point of Junction ,of-the resistor 56 and the lead 22-andthe terminal 8| being the point of junction of the resistors 51 and 58.The resistors 56, 51, and

' 58 have been so chosen that when the needle I5 is at rest the ratio ofthe resistance "to the resistance 51 is equal to the ratio of theelectrolytic resistanc to the resistance 56. It can readily be shownthat the bridge is balanced, that is, the potential difierence resultingfrom the source 24 and applied across the terminals 60 andil will causeno potential difference to exist between terminals 10 and II, theterminal I0 being the point of junction of the resistor 58 and of thelead-2|, and the terminal II being the point of junction of theresistors and 51. The terminals 10 and II are connected through theleads25, 26 to the loudspeaker 30.

It will be understood that the groove undulations in the moving recordwill cause the distance between the needle I5 and the spring I6 toalternately decrease and increase. When the distance decreases, thebridge 50 becomes unbalanced and a'D. C.-voltag appears across theoutput leads 25 and 26., It'can be readily appreciated that themagnitude of this voltage represents the de- Consequently, when distancebetween the needle l5 and the spring I6 continues to decrease theunbalance of th bridge continues and the voltage-across the output leads25, 26 becomes larger. When, however, the deviation of the needle hasattained a minimum and then increases by passing through the normalequilibrium position, the voltage across the output leads 25, 26decreases and reaches the value zero. It is readily apparent that whenthe needle deviates in the opposite direction, the unbalance voltageacross the output leads 25 and 26-appears again with a polarity which isopposite to the one referred to above. Consequently, the polarity of theunbalance voltage appearing across the output leads 25 and 26 representsthe left or right deviation of the needle and the magnitude of thevoltage represents the magnitude of the deviation.

It is, therefore, apparent that across the output leads 25 and 26, weobtain electrical oscillations which represent mechanical vibrations towhich the needle I5 is subjected by the undu1ations of the recordgroove. These oscillations are subsequently impressed to the loudspeaker30 7 any other II will vary, and will cause variations in theelectrolytic resistance between needle and the spring. Consequently,changes in electrolytic resistance between the connecting leads 2I and22 will be produced when the needle I5 becomes subjected sistors hasbeen confined thereforeto applications in which these detrimentalfeatures were not objectionable or'for which they possessed counter E.M. F. is effective on the electrodes owing to the formation of anoxy-hydrogen element. This counter E. M. F. must be annulled by removingimmediately the oxygen when being formed on the electrode. This ispossible by reducing the separated oxygen with the aid oihydrogen.

Accordingly, in the present application the polarization is absolutelyeliminated by the fact that at least one electrode on which oxygen woulddeposit or both electrodes, during the passage of the electric current,are brought into contact with a substance which possesses a hydrogenpressure sufiicient to reduce the oxygen depositing on either electrode.

All solid or liquid substances having any partial pressure of thehydrogen are, therefore, suitable for the elimination of th polarizationefiect; however, such substances are preferably employed which do notcause any ionic reactions afiecting the conductivity that isto say,substances of neutral molecules having a certain partial pressure of thehydrogen, such as for instance, quinhydrone. Besides, allreduction-oxy-, dation systems which present a partial pressure of thehydrogen are suitable for this purpose such as, for instance,indophenol, methylene blue, etc. Furthermore, electrolytes may beemployed such as an'aqeous solution of a mixture of F'eCla and FeClz.Since only very small quantities are,

as a rule, necessary for the reduction of the oxygen, they can besoarranged that the conductivity caused by the addition of suchelectrolytes may be practically neglected with respect to theconductivity to be measured.

In practice, it has been found that slight traces of quinhydrone whichare added to the solution under test, prevent completely polarizationwhen using platinum electrodes. For instance, an amount less than 1 mg.substance is sufficient for one liter of solution. Instead of platinum,also less noble metals, such as gold, silver, iron-nickel alloys, etc.may be employed as substances for electrodes which absorb the oxygendepositing on the electrode in such a manner that the oxygen reacts withthe hydrogen of the material added thereto.

The electrolyte to be used in the electrolytic pick-up may also be ofthe type described in Research paper R. P. 1126, part of the Journal ofResearch of the National Bureau of Standards, vol. 21 (August 28, 1988),which is used in combination with copper electrodes and consistsessentially of a mixture of approximately 1 part 36% aqueous solution 1hydrochloric acid, -'/8' to 4 parts ethyl alcohol and about 2 /2 gramsper 100 ml. of cuprous chloride. V

Instead of employing electrodes and an electrolyte which undergo nochange in their chemical constitution by the passage of the current,electrodes and an electrolyte in which such change does occur may beemployed, but in this case it is desirable to select the electrolyteconnected to the output leads 2|, 22 or the pickup device I0 and to beenergized by an A. C. source I. The network shown in Fig. 4 has itsoutput-leads 25, 26 connected to the loudspeaker 30 and is designed tosubstitute the network 23 in the phonograph assembly shown in Fig. 1.

The electrolyte contained in the pick-up used with the network of Fig. 5may consist of copper sulphate and the electrodes of copper plates or ofmercury nitrate with mercury electrodes, or any other suitablecombination of electrolyte and electrodes.

The network shown in Fig. 5 includes a Wheatstone bridge 0, the threearms of which are made up by resistors I4I, I42, and I43, and the fourtharm of which is constituted by the electrolytic resistance of thepick-up inserted between the leads 2! and 22. The bridge is suppliedwith an A. C. current by means of a source 5 having a frequency f.

It is apparent that when the needle l5 occupies a neutral position thevoltage across the output terminals of the bridge I40 is zero. Whenever,the needle I5 deviates from its neutral position, there appears avoltage across the output terminals of the bridge, the frequency ofwhich is f and the amplitude of which is proportional to the amount ofthe displacement of the needle. Consequently, when the displacement ofthe needle continuously varies due to the vibratory motion, theunbalance voltage appearing across the output terminals of the bridgeI40 is being represented by a modulated carrier, the frequency of thecarrier being the same as that of the source I44 and the modulationvarying in accordance with the vibration of the needle I5. It is desiredto reproduce the vibration of the needle and I am accomplishing this bydemodulating the unbalance voltage derived from the Wheatstone bridge,Thus, the instantaneous value of such a demodulated voltage willrepresent at any instant the magnitude of the displacement of the needleI5 from its neutral position and the polarity of this voltage willindicate at any instance whether or not the 1mpressed displacement hasbeen efiected in the right or in the left direction. In order toaccomplish this purpose, I am applying the output terminals of thebridge I40 to a ring modulator contained within the dotted rectangleI60. The ring modulator IE0 is provided with two pairs of inputterminals I4'Ia, I4Ib, and I'I5a-, I'I5b, respectively, and one pair ofoutput terminals 25 and 20.

The input terminals I 41a and I4'Ib are connect rier telephone system-s,the BellSystem Technical Journal, vol, XVlll, 1939, pp. 315-337.

The type of the circuit contained in the dotted rectangle I isillustrated in Fig. 20, page 318 of the said article. The ring modulatoris essentially a double balanced modulator, By double balanced is meanta modulator in which each input is balanced out from the output, and theoutput contains therefore substantially only the modulation products.

Consider now the ring modulator circuit contained in the rectangle I00.The circuit shown therein comprises a bridge circuit consisting of fourrectiflers I8I, I82, I88, I each of the'said rectifiers constituting aseparate arm 01' the bridge circuit and arranged so that the current canflow only in an anticlockwise direction. The upper corner of the bridgeI85 and the lower corner of the bridge I88 are respectively connected tothe input terminals Illa, Illb and are also connected one to another bymeans of equal resistances I81 and I88 in series. The other corners ofthe bridge I88 and I88 are respectively con. nected to the outputterminals 25, 28 and are also connected one'to another by a pairot equalresistances I8I, I82 in series. The other input terminals Il5a, Il5b ofthe ring modulator are respectively connected to the point I88connecting the resistances I81, I88 and to the point I8l connecting theresistances I8I, I82.

With the circuit as described, current derived from the terminals Illa,Illb may flow either through the rectiflers I82, I83 or through therectifiers I8l, I8I depending upon its direction, but it can never flowthrough all the four rectifiers at the same time, since the rectifiersI82, I83 on one hand and the rectifiers I8l, I8I on the other hand arearranged to flow in opposite the terminal "51). Then one part of thecurrent tends to flow from the terminal 5:: to the terminal I83 andthrough the resistor I 81 to the terminal I85 and then through therectifier I82 and through the resistor I8I back to the terminal I151).The other part of the current tends to flow from the terminal Il5a tothe terminal I83 resistor I 8| is increased as compared to the currentflow through the resistor I82. Thus, the volta e drop across theresistor I8I becomes greater and a positive overall voltage is developedacross the terminals 25, 28. It can be also readily seen that when theunbalance voltage applied across the terminals Illa, Illb increases inmagnitude the corresponding resultant voltage obtained across the outputterminals 25, 28 increases in magnitude also.

It can be shown that if the polarity of the first input voltage wouldreverse with respect to the second input voltage, i. e., if we supposethat the terminal Illa becomes positive as compared to the terminal "521and the terminal Illa becomes negative as compared to the terminal Illb,then the balance becomes oilset in the opposite direction, because thereis a situation where the current flow through the resistor I8I isdecreased as compared to the current flow through the resistor I82. Thenthe voltage drop across the resistor I 8| becomes smaller and theoverall voltage developed across the output terminals 25, 28 has apolarity opposite to the case described above.

It can also be shown that in an intermediate situation where the sgnalvoltage derived. from the input terminals Il5a, Il5b is displaced by '90and through the resistor I88 back to the terminal I88 and through therectifier i8l and through currents flowing through the resistors I8I,I82 are equal and of opposite directions.

It is apparent that under the conditions described in the proceedingparagraph, the polarity of the terminals I85, I86 will be positive withrespect to the polarity of the terminals I88, I80. Consequently,positive voltages are applied to I84. Therefore, these rectifiers willlose their ability of rectifying currents, and

' will allow currents to traverse themin both directions. At the sametime negative voltages are applied to rectifiers I8I, I 88.Consequently, the rectiflers I8I, I83 will retain their rectifyingability and will block currents attempting to traverse them in thenegative direction. Therefore, the rectifiers I82 and I84 are conductivein the resistor I82 back to the terminal Il5b, The

both directions and consequently,.when the unbalance voltage is beingdeveloped across the terminals Illa, Illb, we find that a current tendssituation where the current flow through the degrees with respect to thevoltage derived from the input terminals Illa, Illb then the D. C. volt-'age output from ring modulator is zero.

Let us designate the voltage applied across the terminals Il5a, Il5b asE sin 21 it. Then the unbalance voltage derived from the Wheatstonebridge and applied across the terminals Illa, I llb can be representedbya carrier of the frequency 1, modulated by the vibration of the needleI5 according to a function A(t) which, represents the variation 01' theinstantaneous. displacement of the needle I5 with respect to its neutralposition. qonsequently, the unbalance voltage applied to\ the terminalsIlla, Illb can be represented by the following expression: A(t) sin 21rit. It can be readily appreciated that under these conditions thevoltage derived from the output terminals 25, Not the ring modulatorvaries with time as the function A(t), i. e., this voltage representsthe vibration of the needle. The voltage derived from the ringmodulatoris subsequently applied to the loudspeaker 30 in which the vibrations ofthe needle are being reproduced.

I claim:

v 1. In a phonographic pick-up device, adapted for the translation ofmechanical oscillations from the undulating groove of a moving recordinto corresponding electrical oscillations and including vibrationtranslating means: the combination of a stylus adapted to engage in andto be vibrated by the groove of said record and to infiuence thetranslating means, the translating means comprising a vessel containingelectrically conductive fluid with electrodes immersed in said fluid,and means responsive to the vibrations of said stylus for varying therelative position of said electrodes as a function 01' said vibrationswhereby the resistance of said fluid between said electrode varies, anelectric network connected to said electrodes and a source of voltageconnected to said network whereby the variation of said resistancebetween said electrodes produces electrical oscillations in the outputof said network, said oscillations representing the undulations in thegroove of said moving record.

2. In a phonogr'aphicpick-up device, adapted for the translation ofmechanical oscillations Irom the undulating groove of a moving recordinto corresponding electrical oscillations andineluding vibrationtranslating means: the combination of a stylus adapted to engage in andto be vibrated by the groove of said record and to influence thetranslating means, the translating means comprising a vessel containingelectrically conductive liquid with metallic electrodes immersed in saidliquid, said liquid comprising in solution a substance having in itschemical coinposition the same metal as that of which ,one of saidelectrodes consists, and means responsive ,to the vibrations of saidstylus for varying the relative position of said electrodes as afunction of said vibrations whereby the resistance of said liquidbetween said electrodes varies, an electric network connected to saidelectrodes and a source of voltage connected to said network whereby thevariation of said resistance between said electrodes produces electricaloscillations in the output of said network, said oscillationsrepresenting the undulations in the groove of said 'moving record 3. Ina phoncgraphic pick-up device, adapted for the translation of mechanicaloscillations from the undulating groove of a movingrecord intocorresponding electrical oscillations and in-- cluding vibrationtranslating means: the combination of a stylus adapted to engage in andto be vibrated by the'groove of said record and to influence thetranslating means, the translating means comprising a. vessel containinga conductive fiuid, electrodes immersed in said fluid in a determinedspaced relationship one from the other, whereby the resistance betweensaid electrodes has a determined value corresponding to saidrelationship, electric circuit elements connected to said electrodes toform a bridge network, a supply of current connected to said network forenergizing said network, the values of said circuit elements being suchas to balance said network for said determined spaced relationshipbetween said electrodes and thereby produce no voltage in the outputofsaid network, means responsive to the vibrations of said stylus forvarying the relative position of said electrodes as a function of saidmovements whereby the resist--

